204 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



There can be little doubt, however, that the Devonian and Carboniferous beds 

 once covered the Cambrian rocks through all, or nearly all, of this distance. 

 The minimum thickness given to the younger formations — 2,000 dr feet — is 

 such that we may well believe that the original thickness of the Devonian and 

 Carboniferous combined was, at the Forty-ninth Parallel, of the same order as 

 that determined by McConnell for the contemporaneous strata on the Canadian 

 Pacific railway (main line) section. In the vicinity of Banff he found excellent 

 exposures, giving a total thickness of 6,600 feet. Prom that section northward 

 through all British Columbia and Yukon, and on to northern Alaska, this 

 wonderfully persistent group of rocks may be followed; such breaks as occur in 

 the outcrops through the long traverse are nowhere sufficient to make us doubt 

 that these later Paleozoic strata retain much of their great thickness all the 

 way to Arctic waters. 



In the Little Belt mountains 2,425 feet of beds referred to the Devonian 

 and Mississippian are recorded in the text of the Little Belt Mountains folio 

 (by W. H. Weed). At Mt. Dearborn, Montana, Walcott found more than 

 3,000 feet of Carboniferous limestone.* The Eureka district affords 11,000 feet 

 of contemporaneous rocks, largely limestone. § In the Bisbee district of 

 Arizona, Ransorne found about 1,000 feet of such rocks. f 



In all of the sections above-mentioned there seems to be perfect conformity 

 between the Devonian and Carboniferous, except possibly in parts of Alaska. 

 In the Grand Canyon (Arizona) section about 1.000 feet of Devonian and 

 Mississippian are represented, with an unconformity between them, just as 

 the same region shows unconformity between the Middle Cambrian Tonfco 

 formation and the underlying Chuar series, both of which are conceivably 

 of Cambrian age. 



In the Black Hills of Dakota and in Wyoming the Devonian is wanting 

 and the Mississippian is very thin, though its occurrence there is significant. 



Without going into the details of the many other measured sections on the 

 American side of the Boundary line, the writer will state his belief that the 

 facts of Cordilleran geology show the Devonian and Mississippian formations 

 to form an organic part of the Rocky Mountain Geosynclinal from one end of 

 it to the other, thus once covering practically the entire area of the Eastern 

 Geosynclinal Belt. The Rocky Mountain Geosynclinal was somewhat wider 

 during the Devonian than in the long period represented by the Lower Cam- 

 brian and the Belt terrane. The Mississippian represents a still wider trans- 

 gression of the sea beyond the earlier limits of the down-warp. This early 

 Carboniferous transgression was analogous to that of the Middle Cambrian 

 (Flathead time). The former was so extensive as to make it very difficult, if 

 not impossible, to draw even a rough map of the geosynclinal area for the 

 period. For orogenic theory this partial and irregular drowning of the old 



*C. D. Walcott, Smithsonian Misc. Collections, Xo. 1812, 1908, p. 200. 

 5 A. Hague, Geology of the Eureka District, Monograuh 20, U.S. Geol. Survey, 

 1892, p. 13. 



f F. L. Ransome, Bisbee folio, U.S. Geol. Survey. 



